What the situation with his family? Is he close to them?
Are they aware of his plans?
If my son pulled a stunt like this I’d metaphorically cane his backside so hard he’d not sit down for a month.
If I was you I’d write him a letter whilst he is out tonight and make clear the gravity of the situation.
As you say it’s gone way beyond a stag do ands it seems clear that he is clueless (or in denial/doesn’t care) about the implications of his behaviour….not just re: the birth but your relationship.
You can’t trust him anymore. Not just in being there and having your back but just as bad in him pushing this back on you.
He’s laying the ground for a no win scenario for you either way and pretending to be the injured party.
He goes and all is well - “I told you so, don’t doubt my judgement again”.
He doesn’t go and you give birth later - “I told you so and it’s your fault I missed the stag do”.
He goes and you go into labour - “It’s not my fault the flights were booked up and besides you had your mum to look after you so what’s the big deal”.
You give birth before he goes - “Well your mum is here to look after you”.
All of which miss the point that he chose to have a baby with you. That in doing so he committed to supporting the person (you) who is quite literally carrying the load of this pregnancy.
His part in this process is actually very, very small, yet he can’t even step up to do that. Worse, that he doesn’t want to.
What sort of man would risk missing the birth of his child to go partying abroad?
The answer to that question is not one that any woman with any self respect would want to be with.
He thinks he holds the cards and this is about him.
It’s not. From now you act in the best interests of yourself and your baby. He is of no consequence.